Field of the Invention and Related Art
The present invention relates to an electroconductive device including a carrier transporting layer comprising a liquid crystal compound, particularly a liquid crystal compound which has at least two π-electron conjugated systems, each comprising two fused 6-membered rings and having at least one nitrogen atom in the π-electron conjugated systems.
An applied study on bar-shaped liquid crystal materials has been extensively conducted principally in the field of display devices.
In recent years, by utilizing a self-organization property or alignment structure of a liquid crystal, the liquid crystal is provided with an electroconductivity to be used in a carrier transporting layer, such as an electron transporting layer or a hole transporting layer. For example, Hanna et al. has reported that a smectic liquid crystal is applicable to a material for a carrier transporting layer (Japanese Laid-Open Patent Application (JP-A) 10-312711 and JP-A 9-316442, and Ohyo Butsuri, Appl. Phys., vol. 68, No. 1 (1999)). According to the report, a certain smectic liquid crystal exhibits a carrier mobility of ca. 10−2 cm2/V.sec, which is a maximum value among those given by organic compounds except for molecular crystals and also corresponds to that of amorphous silicon.
Further, as is generally known, in the case of a crystal assuming a polydomain, its domain boundary functions as a larger trap level (carrier conduction obstacle), thereby hindering carrier transport. On the other hand, in the case of a certain liquid crystal (a compound exhibiting mesomorphism), it has been confirmed that its domain boundary does not function as a trap level, whereby carrier transport function is not hindered.
Examples of such a liquid crystal compound may include the following compounds. 
Both the above compounds exhibit a carrier mobility of ca. 10−3 cm2/V.sec.
Since T. W. Tang et al. substantiated in 1987 that it is possible to effect high brightness luminescence under the application of a low DC voltage by utilizing a lamination structure comprising a film of a fluorescent metal chelate complex and a diamine-based molecular film, an applied study on an organic electroluminescence (EL) device as a luminescence device with high speed responsiveness and high efficiency has been extensively conducted. The organic EL device is a self-light emitting device of a carrier injection type using luminescence occurring at the time of recombination of electrons and holes which reach a luminescent layer.
FIG. 4 shows a structure of an ordinary organic EL device.
Referring to FIG. 4, the EL device includes a transparent substrate 25, and thereon layers of a transparent electrode 24, a hole transporting layer 23, a luminescent layer 22 and a metal electrode 21 successively disposed in this order. Between the metal electrode 21 (as a cathode) and the transparent electrode 21 (as an anode) for taking out emitted light, organic compound layers comprising the luminescence layer 22 and the hole transporting layer 23 are formed and disposed each in a thickness of ca. several hundred Å. Examples of the cathode metal electrode 21 may include a metal or an alloy having a smaller work function, such as aluminum, aluminum-lithium alloy and magnesium-silver alloy. Examples of the anode transparent electrode 24 may include an electroconductive material having a larger work function, such as ITO (indium tin oxide). The organic compound layer in this structure (FIG. 4) has a two-layer structure comprising the luminescence layer 22 and the hole transporting layer 23.
FIG. 5 shows another structure of an ordinary organic EL device.
Referring to FIG. 5, the EL device includes a transparent substrate 36 on which a transparent electrode 35 (anode), a hole transporting layer 34, a luminescence layer 33, an electron transporting layer 32 and a metal electrode 31 (cathode) are sequentially disposed in this order. In this case, an organic compound layer has a three-layer structure comprising the electron transporting layer 32, the luminescence layer 33 and the hole transporting layer 34.
Generally, the hole transporting layer (e.g., 23 in FIG. 4 and 34 in FIG. 5) has a function of efficiently injecting holes from the anode (transparent electrode 24 or 35) into the luminescence layer (22 or 33). On the other hand, the electron transporting layer (e.g., 32 in FIG. 5) generally has a function of efficiently injecting electrons from the cathode (metal electrode 31) into the luminescence layer 33.
These hole transporting layer and electron transporting layers also have an electron (carrier) blocking function and a hole (carrier) blocking function, respectively, thus enhancing a resultant luminescence efficiency.
For these carrier (hole and electron) transporting layers, it is important to exhibit a sufficient charge (carrier) transporting ability, particularly a carrier mobility.
Generally, an organic compound in an amorphous state exhibits a carrier mobility of ca. 10−5 cm2/V.sec which is insufficient for carrier transport. Accordingly, if the mobility in the carrier transporting layer is increased, more carriers can be injected into the luminescence layer to enhance the luminescence efficiency. In addition, the higher mobility is also effective in increasing a thickness (e.g., ca. 1 μm) of the carrier transporting layer (generally, several hundred Å-thick). As a result, it becomes possible to prevent an occurrence of short circuit and improve productivity.
For this reason, at present, a compound (material) for the carrier transport layer has been extensively developed in order to achieve a high luminescence efficiency of the organic EL device. In this regard, in order to provide a higher mobility, it has been practiced to impart mesomorphism to an organic compound for the carrier transport layer as described above. However, the resultant carrier mobility is still insufficient.
Incidentally, as a liquid crystal material exhibiting high speed responsiveness to an applied voltage (i.e., a good switching performance) for use in various liquid crystal devices, many liquid crystal compounds have been known. Examples of this may include those described in, e.g., U.S. Pat. No. 5,695,684 (JP-A 7-309838).